Neurofibromatosis type 1 (NF1) is a relatively common inherited disease syndrome caused by germline mutations in the NF1 gene. About one third of NF1 patients develop diffuse, plexiform neurofibromas that can transform to a malignant peripheral nerve sheath tumor - a cancer that is frequently fatal. Remarkably, in human tumors and in mouse models of NF1, neurofibromas almost invariably contain Nf1-null Schwann cells and Nf1 heterozygous mast cells. Transplanting such NF1-prone mice with wild- type bone marrow prevents tumorigenesis, implying that bone marrow derived cells such as mast cells are a required component in pathogenesis, and that targeting signaling pathways in either Schwann cells or mast cells might be of therapeutic benefit. The NF1 gene encodes a large protein with GTPase Activating Protein (GAP) activity towards Ras. Complete or hemizygous loss of the NF1 gene leads to increased Ras activity in both Schwann cells and mast cells, with concomitant activation of downstream effectors that promote proliferation and changes in cell shape and movement. Recently, we have shown that p21-activated kinases play an important role in activating an Erk-mediated proliferation and a p38- mediated motility pathway downstream of Ras in Nf1-deficient mast cells. We postulate that loss of Pak function will diminish activation of key Ras effector pathways in Nf1-deficient Schwann cells as well, and thus could benefit patients with NF1. We propose three aims: 1) We will identify the key substrates of group A Paks in mast cells that affect cell motility;2) Using pharmacologic and genetic means to disable Paks in Schwann cells derived from Nf1-deficient mice, we will determine if loss of Pak function reverses the activation of MAPK and cytoskeletal alterations in vitro;and 3) We will cross Krox20-cre/Nf1flox/- mice, which develop malignant peripheral nerve sheath tumors, with Pak1 knock out mice, or transplant such NF1 mice with Pak1-/- bone marrow cells, to determine if loss of Pak1, globally or in bone marrow derived cells, affects disease progression. The proposed studies will not only increase our understanding of cardinal cancer-relevant signaling pathways, but could establish the Paks as suitable targets for therapeutic intervention in this otherwise untreatable disease. PUBLIC HEALTH RELEVANCE: p21-activated kinases (Paks) are key regulators of signaling downstream of Ras, a protein that is activated as a result of loss of the NF1 gene. Currently, there are no effective therapies for NF1-related malignancies. We have developed genetic models for studying Pak function in animals, as well as the first specific chemical inhibitor of these enzymes;for these reasons, we are in a unique position to explore the biological role of Paks in NF1 and to determine if these enzymes represent suitable targets for therapy.